1. Field of the Invention
This invention relates to a method and apparatus for controlling a digital copying machine in which an image is formed on a sheet of paper based on image data obtained from an image input unit.
2. Discussion of the Related Art
Conventional electrophotographic copying machines use a lens to focus light reflected from a document onto a photoreceptor and expose the photoreceptor to form a latent electrostatic image. The latent electrostatic image is then developed and formed into a toner image. The toner image is then transferred onto a sheet of paper to produce a copy of an original document.
These so-called "light-lens" electrophotographic copying machines are known in the art. However, a new type of electrophotographic copying machine, generally referred to as a "digital copying machine," is now attracting attention in the copying machine field. A digital copying machine copies a document image using a sensor-based image input unit to obtain image signals, converts the obtained image signals into digital signals, processes the digitized image signals, and forms an image corresponding to the document image on a sheet of paper using an image output unit such as a laser printer.
In general, the digital copying machine processes image signals from the image input unit in almost real time and feeds the processed signals to the image output unit. Alternatively, digitized image signals are stored in memory on a page basis, and supplied to the image output unit after processing.
To increase copying efficiency, the digital copying machine is often used in combination with an automatic document handler (ADF) or a recirculating automatic document handler (RDH). An example will now be considered in which a digital copying machine equipped with an ADF prepares and sorts n copies of a document consisting of m pages. Copying with an ordinary ADF which is not capable of recirculating documents requires that n copies of each page be successively produced and that the produced copies be sequentially discharged into n bins of a sorter. This operation must be repeated m times to complete n sets of copies having m pages. Copying with an RDH allows the required number of copies to be produced and sorted without using a sorter. That is, m pages of the document are sequentially copied one time and the processed copies are discharged into a discharge tray. Thereafter, the document is recirculated as many times as are required to complete n copies. However, the RDH passes the document through belts and rollers many times, and this repeated handling is liable to damage document pages.
To overcome this problem, a copying apparatus has been proposed in which the image data of the respective document pages are stored in a storing unit and that the stored image data be read in a desired sequence to produce desired copies. For example, all the images in m document pages are read by the image input unit to obtain resulting image signals. The obtained image signals are then converted into corresponding digital signals and stored in a hard disk unit. If the stored digital signals are repeatedly read from the hard disk unit and output in the order of first, second, third, . . . mth page, first, second, third, . . . mth page, and so on, n copies can be prepared and sorted without using a sorter and without handling the original document more than once.
Furthermore, the image output unit of the digital copying machine may be used as a printer for a print job from another image data source, e.g., a work station. The term "job" herein used is intended to mean a block of image data subjected to similar processing. This arrangement requires that the digital copying machine be connected to the other data source through a communication line and that image data from the other image data source be applied to the image output unit during a period in which a document is being read from the image input unit and stored in the storing unit. Upon printing of the job from the other image data source, the stored image data of the document can be read from the storing unit and applied to the image output unit to produce copies. Accordingly, the shared use of the image output unit reduces the need for units which perform similar functions and makes efficient use of the image output unit.
Occasionally, no writable area is available in the storing unit when a larger volume of data is applied from the image input unit to the storing unit than is taken from the storing unit by the image output unit. In such circumstances, a conventional copier unit will cancel the job, and as a result, the unit must wait until the volume of the data stored in the storing unit is reduced before again reading the document.